From: Yu Kuai <yukuai3@xxxxxxxxxx> Changes in v4: - add detailed explanation of backgroud and design in cover letter; - merge some patches; Changes in v3: - rebase with latest md-next; - remove patch 2 from v2, and replace it with a new patch; - fix a null-ptr-derefrence in rdev_attr_store() that mddev is used before checking; - merge patch 20-22 from v1 into one patch; - mddev_lock() used to be called first and can be interruptted, allow new api, which is called before mddev_lock() now, to be interruptted as well; - improve some comments and coding; Changes in v2: - rebase with latest md-next; - remove some follow up cleanup patches, these patches will be sent later after this patchset. ##### Background Our testers started to test raid10 last year, and we found that there are lots of problems in the following test scenario: - add or remove disks to the array - issue io to the array At first, we fixed each problem independently respect that io can concurrent with array reconfiguration. However, on the one hand new issues are continuously reported, on the other hand other personalities might have the same problems. I'm thinking about how to fix these problems thoroughly. Refer to how block layer protect io with queue reconfiguration(for example, change elevator): ``` blk_mq_freeze_queue -> wait for all io to be done, and prevent new io to be dispatched // reconfiguration blk_mq_unfreeze_queue ``` Then it comes to my mind that I can do something similar to synchronize io with array reconfiguration. ##### rcu introduction see details in https://www.kernel.org/doc/html/next/RCU/whatisRCU.html - writer should replace old data with new data first, and free old data after grace period; - reader should handle both cases that old data and new data is read, and the data that is read should not be dereferenced after critical section; ##### Current synchronization Add or remove disks to the array can be triggered by ioctl/sysfs/daemon thread: 1. hold 'reconfig_mutex'; 2. check that rdev can be added/removed, one condition is that there is no IO, for example: ``` raid10_remove_disk if (atomic_read(&rdev->nr_pending)) err = -EBUSY; ``` 3. do the actual operations to add/remove a rdev, one procedure is set/clear a pointer to rdev, for example: ``` raid10_remove_disk p = conf->mirrors[xx] rdevp = &p->rdev/replacement *rdevp = NULL ``` 4. check if there is still no io on this rdev, if not, revert the pointer to rdev and return failure, for example: ``` raid10_remove_disk synchronize_rcu() if (atomic_read(&rdev->nr_pending)) err = -EBUSY *rdevp = rdev ``` IO path is using rcu_read_lock/unlock() to access rdev, for example: ``` raid10_write_request rcu_read_lock rdev = rcu_dereference(mirror->rdev/replacement) ... rcu_read_unlock raid10_end_write_request rdev = conf->mirrors[dev].rdev/replacement -> rdev/rrdev is still used after rcu_read_unlock() ``` ##### Current problems - rcu is used wrongly; - There are lots of places involved that old rdev can be read, however, many places doesn't handle old value correctly; - Between step 3 and 4, if new io is dispatched, NULL will be read for the rdev, and data will be lost if step 4 failed. ##### New synchronization Similar to how blk_mq_freeze_queue() works Add or remove disks: 1. suspend the array, this should guarantee no new io is dispatched and wait for dispatched io to be done; 2. add or remove rdevs to array; 3. resume the array; IO path doesn't need to change for now, and all rcu implementation can be removed. There are already apis to suspend/resume the array, unfortunately, they can't be used here because: - old apis only wait for io to be dispatched, not to be done; - old apis is only supported for the personality that implement quiesce callback; - old apis can only be called after the array start running; - old apis must hold 'reconfig_mutex', and will wait for io to be done, this behavior is risky because 'reconfig_mutex' is used for daemon thread to update super_block and handle io. In order to prevent potential problems, there is a weird logical that suspend array hold 'reconfig_mutex' for mddev_check_recovery() to update super_block, however, this is still not enough; Then main work is divided into 3 steps, at first make sure new apis to suspend the array is general: - make sure suspend array will wait for io to be done(Done by [1]); - make sure suspend array can be called for all personalities(Done by [2]); - make sure suspend array can be called at any time(Done by [3]); - make sure suspend array doesn't rely on 'reconfig_mutex'(PATCH 3-5); The second step is to replace old apis with new apis(PATCH 6-16): ``` From: lock reconfig_mutex suspend array resume array unlock reconfig_mutex To: suspend array lock reconfig_mutex unlock reconfig_mutex resume array ``` Finally, for the remain path that involved reconfiguration, suspend the array first(PATCH 11,12, [4] and PATCH 17): ``` From: // reconfiguration To: suspend array // reconfiguration resume array ``` Preparatory work: [1] https://lore.kernel.org/all/20230621165110.1498313-1-yukuai1@xxxxxxxxxxxxxxx/ [2] https://lore.kernel.org/all/20230628012931.88911-2-yukuai1@xxxxxxxxxxxxxxx/ [3] https://lore.kernel.org/all/20230825030956.1527023-1-yukuai1@xxxxxxxxxxxxxxx/ [4] https://lore.kernel.org/all/20230825031622.1530464-1-yukuai1@xxxxxxxxxxxxxxx/ Yu Kuai (19): md: use READ_ONCE/WRITE_ONCE for 'suspend_lo' and 'suspend_hi' md/raid5-cache: use READ_ONCE/WRITE_ONCE for 'conf->log' md: replace is_md_suspended() with 'mddev->suspended' in md_check_recovery() md: add new helpers to suspend/resume array md: add new helpers to suspend/resume and lock/unlock array md/dm-raid: use new apis to suspend array md/md-bitmap: use new apis to suspend array for location_store() md/raid5-cache: use new apis to suspend array md/raid5: use new apis to suspend array md: use new apis to suspend array for sysfs apis md: use new apis to suspend array for adding/removing rdev from state_store() md: use new apis to suspend array for ioctls involed array reconfiguration md: use new apis to suspend array before mddev_create/destroy_serial_pool md: cleanup mddev_create/destroy_serial_pool() md/md-linear: cleanup linear_add() md/raid5: replace suspend with quiesce() callback md: suspend array in md_start_sync() if array need reconfiguration md: remove old apis to suspend the array md: rename __mddev_suspend/resume() back to mddev_suspend/resume() drivers/md/dm-raid.c | 10 +- drivers/md/md-autodetect.c | 4 +- drivers/md/md-bitmap.c | 18 ++- drivers/md/md-linear.c | 2 - drivers/md/md.c | 233 ++++++++++++++++++++----------------- drivers/md/md.h | 43 +++++-- drivers/md/raid5-cache.c | 64 +++++----- drivers/md/raid5.c | 56 ++++----- 8 files changed, 226 insertions(+), 204 deletions(-) -- 2.39.2
